Heat exchanger and method for making the same

ABSTRACT

A high effectiveness compact heat exchanger having internal passageways containing substantially transverse foraminous elements of high heat conductivity includes heat-flowed passageway walls of synthetic material that are interspersed into the interstices of the foraminous elements. Metal screens of fine mesh are contiguously stacked to provide transverse heat conduction relative to longitudinal flow paths. Internal plastic barriers transverse to and at least partially penetrating the screens define composite passageway walls providing leak free construction capable of sustaining substantial pressure differentials between adjacent passageways. The passageways are varied in cross-sectional area along their lengths, as by incremental displacements of successive barriers, to provide flow equalization for maximum efficiency.

United States Patent 1191 Cowans July 23, 1974 HEAT EXCHANGER AND METHODFOR 3,409,075 11/1968 Long res/154 MAKING THE SAME 3,477,504 11/1969Colyer et al.... 165/164 3,491,184 l/l97O Rleldljk 165/165 X [76]Inventor: Kenneth W. Cowans, 3118 Patricia L05 Angeles Cahf' PrimaryExaminer-Albert W. Davis, Jr. 22 Filed; 13, 1972 Attorney, Agent, orFirm-Fraser and Bogucki [21 Appl. No.. 234,129 ABSTRACT Related ApphcamnData A high effectiveness compact heat exchanger having [62] Division-ofSer. No.3,395,Jan..16', 1970, abandoned. internal passageways containingsubstantially trans verse foraminous elements of high heat conductivity[52] US. Cl 165/146, 165/141, 165/165, includes h at-flowed assagewaywalls of synthetic j 165/179 material that are interspersed? into theinterstices of [51] Int. Cl F281 13/06 the foraminous Metal screens offine h [58] Field of Search 165/179, 164, 165, 1 are contiguouslystacked to providetransverse heat 1 1 14. 147, 141 conduction relativeto longitudinal flow paths. Internal plastic barriers transverse to andat least partially pen- [56] References g w etrating the screens definecomposite passageway UNITED STATES PATENTS walls providing leak freeconstruction capable of sus- 1,734,274 11/1929 Schubertm' 165,166 xtaining substantial pressure differentials between adja- 1,825,3219/1931 La Mont et a1 .5165/147 cent passageways. The passageways arevaried in 2,062,321 12/1936 Levin 165/140 X cross-sectional area alongtheirlengths, as by incre 2,460,359 2/1949 Tfumplerm 165/14l X mentaldisplacements of successive barriers, to pro- 2,466,684 4/1949 Case165/147 X i fl equalization for maximum ffi 1 2,716,333 8/1955Collins... 165/141 X 3,228,460 111966 3 Claims, 14 Drawing FiguresGarwin 165/179 X councuous 'PAIEIIIEIIIIIIzaIIII SHEET 1 0F 7 I I I I II I I I I I I I HELIUM SOURCE Umm Sm; 57

OUTLET SYSTEM 24 I DESICCANT CHAMBER Fl G I PATENTED I 3. 825.063

sum 2 or 7 PROCESSED GASES l0 CONTIGUOUS SCREEN ELEMENTS 53 2 M0 00PRECIPITAfION zone LENGTH FROM HOT END FIG.5

PATENTEnamzsxsm SHEET 5 0F 7 FORN PLASTIC ELEMENTS PRECLEAN PLASTIC FORMSCREEN ELEMENTS PRECLEAN SCREENS FIG F. N N N m mm m wm 0 m Mm Em r...n- S 0 HL P R RI: Rlns A In I. AL m w m m n wmwm nu N N A c m w M Hm n IPmEmsmmz 3.825.083

sum- 6 OF 7 HIGH PRESSURE HELIUN SOURCE CRYOGENIC REFRIGERATOR cormcuousSCREEN ELEMENTS HELIUM HELIUH PATENTEDJULZBIBM 3,825,063 SHEET 7 BF 7councuous SCREEN ELEMENTS 1 HEAT EXCHANGER AND METHOD FOR MAKING THESAME This is a division of application Ser. No. 03,395, filed Jan. 16,I970.

BACKGROUND OF THE INVENTION This invention relates to heat exchangersand systems using heat exchange relationships, and particularly to metaland composite heat exchangers of high heat transfer effectiveness andmethods of making the same.

In many modern applications of heat exchangers particularly stringentoperative design requirements must be met in terms of heat transfereffectiveness, compactness, structural properties and similar factors.The extreme demands that may be imposed on heat exchangers areexemplified by life support systems such as those disclosed in aco-pending patent applicgtion, as-

same time imposes substantial thermal gradients and stresses.

It is known to construct heat exchangers having transverse elements forlateral heat distribution between passageways, as shown by US. Pat. No.3,228,460. As discussed in that patent, transverse perforated plates maybe separated byapertured spacers in a laminate, with the spacers beingconfigured todefine passageways for counterflowing fluids, and theplates facilitating heat interchange. An alternative construction isshown in an article entitled A New Type of Compact HeatExchanger with aHigh Thermal Efficiency," by G. Vonk, pp. 582-589 of Advances inCryogenie Engineering, Vol. 13 (Plenum Press, New York 1967). In thisunit, the transverse conductive elements are copper screens and theinsulative elements are resin-impregnatedpaper having punched holeswhich define the passageways. The aforementioned heat exchangerconstructions are primarily for the particular applications described,whereas greater versatility is often desired. For example, whereparticular thermodynamic or chemical characteristics are to be achievedduring heat exchange, it may be desirable to have different temperaturegradients along the length of the exchanger. Selection and control ofthe temperature gradient profileshould not, of course, adversely affectthe other desirable characteristics.

In addition, needs exist for improved methods of fabricating exchangers,and for improved exchangers themselves, in order to reduce costs ofmaterial, labor and equipment without sacrificing reliability or.performance. The term heat exchanger, as used herein, is intended toinclude regenerators, as well as counterflow. concurrent flow, and otherconventional types of exchangers.

SUMMARY OF THE INVENTION The purposes and objects of the presentinvention are achieved by heat exchanger systems comprising a pluralityof transverse heat conductive, filamentary elements that arecontiguously stacked along thelongitudinal axis of the heat exchangerand internal heatflowedwalls of thermoplastic material that penetratethe interstices of the filamentary elements to join them longitudinallyand seal them internally. The passageway walls are varied incross-sectional area along their length to equalize flows within thepassageways.

A specific example of a compact heat exchanger system in accordance withthe invention comprises a cy-.

lindrical body defined by a plurality of contiguous metal meshelernents,disposed about and transverse to a central axis and internally dividedinto separate passageways by non-conductive heat-flowed plastic barriersthat impregnate and seal the meshes while forming composite walls.Although thermal interchange at each incremental region along the lengthof the exchanger is highly efficient, longitudinal conduction is minimaland high heat transfer effectiveness is achieved along with an extremelysharp temperature gradient. The unit can withstand significant pressuredifferentials, permits ready headering of fluids, and may havearbitrarily determined internal configurations. Such constructions maybe used to handle two or more fluids in counterflow or concurrent flowrelation.

The dimensions of the passageways are varied along their lengths forequalization of flows and maximizing efficiency. In such arrangements,successive plastic wall segments have slight relative displacements, butnonetheless overlap, prior to unification. After unification the wallsare continuous but the passageway crosssections are non-uniform. In aspecific example the passageways of one set are concave along theirlengths and the passageways of another are convex along their lengths.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of theinvention may be had by reference to thefollowing description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a combined perspective and schematic view, partially brokenaway, of one example of a heat'exchanger in accordance with theinvention as used in a life support system;

FIG. 2 is a side sectional view of the heat exchanger of FIG. 1;

FIG. 3 is an end sectional view, taken along the line 33.of FIG. 2, andlooking in the direction of the appended arrows;

FIG. 4 is an enlarged fragmentary view of a portion of the heatexchanger of FIGS. 2 and 3;

FIG. 5 is a graph showing a temperature gradient profile along aheat'exchanger constructed as shown in FIGS. l-4;

FIG. 6 is an exploded view of a fragment of a heat exchanger inaccordance with the invention showing a number of elements as they areemployed in the assembly of a unit;

FIG. 7 is a block diagram representation of an example of a process formaking heat exchangers in accordance with the invention;

FIG. 8 is a combined perspective and block diagram view, partiallybrokenaway, of a regenerator in accortion system;

FIG. 9 is a side sectional view of the regenerator of FIG. 8;

FIG. 10 is an end sectional view of the regenerator of FIGS. Sand 9; v

FIG. 11 is a cross-sectional end view of a fragment of an ultra lowtemperature regenerator of different configuration;

FIG. 12 is a side sectional fragmentary view of an alternative wallconstruction in heat exchangers in accordance with the invention;

FIG. 13 is a front sectional view of a heat exchanger constructionutilizing passageway area variations along the passageway length; and

FIG. 14 is an enlarged front sectional view, somewhat idealized, of afragment of the arrangement of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION A heat exchanger 10 in accordancewith the invention is illustrated as employed in conjunction with a lifesupport system of the type disclosed in general terms in patentapplication Ser. No. 623,616 referred to above. The heat exchanger 10and the various aspects and features it provides are utilized incombination with a processor system 12 which includes liquid oxygen 14within a liquid oxygen vessel 16 encompassed by liquid nitrogenl8'within a temperature control cryogen vessel 20. As described in theabove-identified patent application, an expired mixture from one or moreusers at an inlet-outlet system 22 is passed through a desiccant chamber24, then through the heat exchanger 10, in which the expired gases arelowered to the near cryogenic level. In the processor system 12, theliquid oxygen 14 is maintained in a selected pressure range, andthermodynamic equilibrium between the expired gas mixture and the liquidoxygen 14 is utilized to return a gas mixture having a'preciselycontrolled partial pressure of oxygen, constituting a safe breatheablemixture. The gases after processing are passed through the heatexchanger 10 in counterflow relation to the expired gases, for reheatingto near the ambient level. A helium source 26 under pressure operatesthrough an ambient pressure responsive device 28, to make up theremainder of thebreatheable mixture.

The system shown is arranged to freeze carbon dioxide internally withinthe heat exchanger 10, although a separate CO removal device, such as achemical container, may be used alternatively or in addition.

Further details as to the arrangement and operation of the life supportsystem generally represented in FIG. 1 have been omitted for brevity,inasmuch as they are included in the copending application previouslymentioned.

The heat exchanger construction shown in FIGS. 1-4 not only has thedesired compactness and effectiveness for this application, but is anextremely strong and stable unit capable of withstanding extremely highpressure loadings and pressure differentials.

The exchanger 10, in general terms, comprises a plurality ofcircumferential passageway segments disposed substantiallyconcentrically about a central axis of the exchanger, and includes afirst pair of headers 30, 31 at one longitudinal end of the exchangerand a second pair of headers 33, 34 disposed at the oppositelongitudinal end of the exchanger. As will be evident to those skilledin the art, other header arrangements may be utilized, and the headersat each end need not be the same. For convenience of reference the firstend will be referred to as the hot end of the-exchanger 10, this beingthe end in which the incoming gases from the inlet-outlet 22'are at ornear the ambient level, along with the counterflowing gases beingreturned to the inletoutlet system after passage through the heatexchanger. The cold end of the exchanger, 10 is that end at which theinflowing expired gases which have passed through the heat exchanger andthe gases from the processor system 12 which are being directed into theheat exchanger are near the cryogenic level.

. Inasmuch as the header-pairs are alike in the present example, onlythe first pair 30, 31 is described in detail herein. One header of thepair comprises a cylindrical plenum 36 spaced apart from the end of theheat exchanger and interconnected with the interior of appropriatepassageways within the exchanger by hollow tubes 38. A chamber 40 aboutthe tubes 38 comprises the second header, and is interconnected with endapertures 42 which are in communication with other passageways withinthe heat exchanger 10. A side outlet fitting 44 on the chamber 40couples the processed gases to the inlet-outlet system 22, and an endoutlet fitting 46 couples the expired gases into the plenum 36.

l The central body of the heat exchanger 10 is encompassed by a metalcylinder housing 50 that forms the outer wall of the exchanger. Arefrigeration source for the exchanger 10 comprises a hollow metal tube52 wound about a part of the cylindrical housing 50, the thermal contactbeing materially enhanced by solder 53. Boil off or liquid nitrogen 18or other temperature control cryogen in the metal tube 52 therefore isin good thermal contact with the interior of the housing 50, and servesas a heat sink to the. interior gases.

The counterfiow passageways in the interior of the housing 50 aredefined by internal heat-flowed plastic barriers which comprise bothcircumferential segments 55 lying at different radii, and radial walls57 lying at different circumferential positions. In the present example,the expired gas mixture passes from the first end to the second end ofthe heat exchanger 10 in alternate radially spaced ones of thepassageways, and the processed gas mixture flows in the oppositedirection in the remaining alternate passageways. The particularconfiguration shown (best seen in the end sectional view of FIG. 3), isarranged such that the volume of gas flowing in each passageway isessentially equal to the counterflowing gases in the adjacent passagewayor passageways. As will be evident from the approximation of FIG. 3,which is not drawn to scale, a regular progression of sizes does notexist when flow equality is to be maintained in a concentric passagewayconfiguration.

The foraminous structure in this example comprises a plurality of metalmesh or screen elements 59, each of whichlies in a plane transverse tothe central axis of the heat exchanger 10 and has a cross-sectional areathe metal filaments of the mesh elements 59 are disposed within a matrixof heat-flowed material of relatively low thermal conductivity. Athermoplastic, specifically polystyrene, is employed in the presentexample for use in a cryogenic-type system. In accordance with theinvention, the mesh elements 59 are substantially contiguous and in thepresent example are actually in abutment. Despite this contact, thelongitudinal thermal path is so disrupted and discontinuous relative tothe transverse conductive paths that longitudinal conduction isnegligible. The plasticfills the interstices of each mesh, along theinterior walls 55, 57. The wall thickness is not shown to precise scalebut may be substantial, i.e. a considerable fraction of the passagewaywidth, or thin if desired. In the practical example being discussed ofan exchanger for a life support system, the screen elements 59 are bothmesh and 100 mesh aluminum and the structure is approximately 9" longand 5" in diameter. The filling factor, for this configuration, can beextremely high. In this example approximately 110 mesh elements per inchwere utilized, providing open flow paths comprising approximately 58percent of the total area.

As best seen in. the fragmentary view of FIG. 4, the screen elements 59are adjacently disposed along the length of the exchanger 10. Thecross-sectional flow area within the passageways, defined by theinterstices in the elements 59, is sufficiently large to-permit lifesupport operation with a reasonably low pressure gradient between theopposite ends of the exchanger. The filaments interspersed in the flowpath create some turbulent flow, but more importantly conduct heatlaterally, so as to integrate or equalize the temperature in a lateralplane within the exchanger 10. The circumferential and radial plasticwalls 55, 57 respectively inter- I pose virtually no lateral heattransfer barrier, inasmuch as the plastic is fused within theinterstices of the meshes, as well as interconnecting the screens 59 soas to seal off each passageway from the adjacent passageways. i

The arrangement of composite metal-plastic passageway walls inaccordance with the invention frees such structures from requirementsthat the laminate beuniform throughout. For example, in the heatexchanger 10 of FIGS. l-4 the mesh sizes of the screen elements 59 varyalong the longitudinal axis. Specifically, about 40 percent of the totallength of the structure interior of the ends comprises coarser No. 10mesh screen elements 59 than the No. 100 mesh otherwise used. In theportion along which the No. 10 mesh is disposed, heat interchangesbetween the elements 59 and the flowing gases are substantiallydiminished by comparison to the remaining portions, and the temperaturegradient is considerably lowered. Incoming hot gas containing carbondioxide is therefore lowered in temperature to the region, 1 30C. to140C., at which most CO freezing occurs. This shaping of the temperatureprofile is illustrated in FIG. 5, which is a plot of position along thelength of the exchanger 10 as the abscissa, against temperature indegrees Centigrade as the ordinate. It may be seen that the oppositeends of the exchanger 10 have a temperature differential ofapproximately 200C, which is in excess of 10C. temperature change perinch. In the interior, relatively low temperature gradient region,however, the approximately 10C. range in which carbon dioxide solidprecipitation occurs is greatly extended, so that the gradient isapproximately 2C. to 3C. per inch. Thus the'carbon dioxide freezes ments65 and 66 are assembled into a laminate, which is thereafter unified. Asbest seen in FIG. 6, foraminous elements 65 of high heat conductivitymaterial, namely aluminum mesh, are formed to the configuration and sizedesired. These conductive elements 65 are stacked in alternating fashionwith internally apertured plastic sheets 66 which correspond in externalsize and configuration to the foraminous elements 65. In the presentexample, the plastic sheets 66 are of polystyrene, and approximately0.010" inthickness. Appropriate alignment during lamination may beassured by stacking the elements 65, 66 in a fixture if desired. Toinsure that the plastic-elements 66 are added correctly if the assemblyis being fabricated by hand, the elements 66 may have unequally spacedradial barriers which misalign if an element is stacked upside down.

The elements 65, 66 may be cut, punched or molded or prepared in otherconventional fashion. When the laminate has been stacked to a desiredlength, referring now to FIG. 7, it is thereafter unified by a time,temperature and pressurecycle sufficient to cause the nonconductiveelements 66 uniformly to reach the plastic fiow state and to becomeactivated for final cure. In this state, the material flows orimpregnates into the interstices in the conductive elements 65 fillingand sealing the interstices and defining the internal composite barriersor passageway walls..The flow is, however, largely limited to theadjacent mesh areas.

The following examples are illustrative of different heat exchangerfabrication methods.

Example I Aluminum alloy screen woven of 0.0045" diameter wire wasutilized in conjunction with 0.0075" sheet clear polystyrene, each beingcut with steel rule dies to form the appropriate configurations. Theappropriate number of patterned parts were then cleaned as set outbelow:

A. The aluminum screens were degreased in M-l7 solvent, then immersedfor 3 minutes in a solution of deionized water, sulphuric acid (specificgravity 1.84) and sodium dichromate at room temperature, then rinsed incold water by a water spray, then cleaned with ultrasonic agitation inisopropyl alcohol for two minutes, and finally air dried with filteredair.

B. The polystyrene parts were immersed in isopropyl alcohol withultrasonic agitation, followed by air drying with filtered air. Thecleaning of both the aluminum screens and the polystyrene was effectedin clean room facilities, which were thereafter also utilized duringassembly.

Subsequent to the pre-cleaning of the formed ele-- ments, they wereassembled into a laminate prior to bonding and concomitant unification.In the present example, the assembly comprised alternating aluminumscreen elements and apertured polystyrene discs, to provide a finishedcylinder approximately 5" in di- 7 ameter by 9 long. The assembly wascompressed and bonded in the following sequence:

A. Uniform pressure was applied to both ends of the assembled cylinder.The cylinder was placed in a matching sleeve, and pistons slidableinwardly within the sleeve were disposed adjacent each end of thecylinder. A hard vacuum was pulled on the specimen; once a stable vacuumlevel was reached the pistons were clamped in place after being fullyseated. Sintered brass discs permeable to helium were inserted betweenthe ends of the stack and the pistons in the assembly.

B. Preheated helium at approximately 280F. was then flowed through thestack at 0.5 CFM. The preheated helium purged the system of other gasesand brought the assembly closer to the plastic flow temperature.

C. The entire fixture was then immersed in oil maintained at 350F., forthirty minutes to cause plastic v flow of the polystyrene. At the end ofthis time, the helium flow was stopped and a hard vacuum was pulled onthe assembly until a stable vacuum level was again reached. The pistonswere released to be drawn inwardly,further compressing the assembly.

The second compression under these uniform temperature conditionsassured flow of the plastic material into the interstices of the screenelements and maintenance of the densified form.

D. After the second compression, the entire assembly was maintained at35 F for approximately 20 minutes for stabilization.

E. After stabilization, the temperature of the assembly andfixture wasreduced and the assembly was allowed to cool at a slow rate, whilemaintaining flow of helium. After reaching the approximate temperaturelevel of the helium, the assembly was allowed to air cool to ambientconditions.

The assembly was then removed from the fixture for affixation of theheader assemblies.

Example II For the assembly of a structure utilizing copper screens, thecopper screens were woven of 0.010 diameter wire, and precleaned asfollows:

The copper screens were degreased in M-l7 solvent,

immersed for 10 minutes in a solution of ferric sulphate and sulphuricacid (specific gravity 1.84) at 150F., rinsed by a cold water spray,immersed at room temperature in a solution of sodium dichromate andsulphuric acid (specific gravity 1.84) until clean and bright, rinsedagain in a cold water spray, cleaned for 2 minutes in isopropyl alcoholwith ultrasonic agitation, and then air dried with filtered air.

As in Example I, the pre-cleaning, assembly and bonding and compressionsteps were undertaken in clean room facilities. The remaining assemblysteps corresponded to those set out in Example I.

The passageway walls that are formed are substantially the thickness ofthe original wall segments in the non-conductive elements, although aminor amount of lateral spreading may occur. In the given examples, thewall thicknesses were 0.090" for the interior walls and 0.070 for theexterior walls. Those skilled in the art will recognize that the time,temperature and pressure relationships that are to be maintained willvary primarily dependent upon the nature of the thermoplastic or othermaterial that is employed, as well as the type of construction that isdesired. Higher temperatures are generally required for epoxies than forpolystyrene, for example. I

The views of FIGS. 8, 9 and 10 illustrate a regenerator, specifically aunit suitable for employment with a cryogenic refrigerator system. Thecryogenic refrigerator may be of any well known type using athermodynamic cycle in which a regenerator function is essential, suchas a Vuilleumier machine. Successive stages of regenerators are oftenemployed, to achieve the final very low temperatures that are desired insome systems. At final temperature levels in the range from 5K. to 10K.,for example, thelowest temperature regenerator stage must meet criticalrequirements as to thermal mass and heat transfer efficiency. That is,sufficient material of appropriate specific heat must be utilized, andthe heat transfer properties of the unit must transfer heat to and fromthe stream of flowing refrigerant gas with a very low temperaturedifferential (e.g. less than approximately 0. 1K.). The heat transfermust also be achieved with an acceptable ratio of heat transferefficiency'to pressure drop, for the given final temperature desired.The low temperature regenerator 72 shown in FIGS. 8-10 fulfills theserequirements while at the same time being economically and readilyfabricated. Other-regenerator stages have not been shown for simplicityand are here considered to be part of the refrigerator system 70.

The regenerator 72 comprises a metal-plastic exchanger within an outercylindrical housing 73, and including a stack of copper screens 75 withinternal heatflowed longitudinal epoxy barriers 77, defininglongitudinal passageways 78 for the refrigerant gas. In addition,however, the barriers 77 define internal chambers 79 confining a thermalmassmaterial,specifically pressurized helium gas, in stagnant fashion.The conduit system for the flow of refrigerant through the passageways78 has been shown only schematically. The helium gas, typicallypressurized to the approximate range of 20-30 atmosphere, is suppliedfrom a source through conduits 82. The number of flow passageways 78 andstagnant chambers 79 is variable at the choice of the designer, as willbe evident to those skilled in the art. Thus, each refrigerantpassageway 78 may be arranged with individually adjacent chambers 79 forbest heat transfer properties in a specific case. In the presentexample, three refrigerant passageways 78 are disposed along a diameterof the cylinder and two chambers 79 are disposed on each side of thediameter.

The operative characteristics of the regenerator 72 shown by way ofexample adequately satisfy the requirements of ultra low temperaturesystems. The pressurized helium has an appropriate specific heat forthese low temperatures and further provides a thermal mass which has ahigh ratio to that of the flowing refrigerant gas. The ratio of thetotal thermal mass of the regenerator to the total thermal mass of therefrigerant is in the range of 10 to 20, which 'is much more thanadequate. These'factors are made possible by the leaktight, highstrength composite wall constructions which can contain the pressurizedhelium,'together with the highly efficient thermal interchange effectedby the laterally continuous heat conductive paths provided by the copperscreens. The various sources of loss and inefficiency, includingparasitic losses, temperature difference between the refrigerant gas andassociated screen, and total fin losses, can be shown to have only aminor effect on efficiency. v

In constructions in accordance with the invention it is feasible toprovide small passageways and thin walls, so that excellentinterspersion of flow passageways can be achieved with respect to thethermal mass. An example of a fragment of the cross-section of one suchregenerator is shown in FIG. 11, it being understood that thepassageways and stagnant chambers receive gates in the fashion showngenerally in FIGS. 8-10. The refrigerant flows in round passageways 83uniformly dispersed throughout a filament-reinforced composite wallstructure 84. These passageways 83 in this example are approximately0.080 or less in diameter, and may of course have other cross-sectionalshapes and be distributed differently. The helium storage chamberscomprise a number of pairs of curved apertures85, each pair beingconcentric with a different passageway 83. The helium storage apertures85 each have are lengths of slightly less than that of a semicircle, sothat .each pair almost completely encompasses its associated passageway83. The wall thickness between a passageway 83 and its paired heliumstorage apertures 85 is the same as the radial separation, and here isapproximately 0.060" or less. The spacing between the facing ends of theapertures 85 is also the same as the wall thickness. The width of theapertures 85 (again expressed as a radial distance) is approximately0.030" or less.

All of the stated dimensions can be reduced substantially by one-half ormore, when regenerators are constructed in accordance with theinvention. The composite walls still have adequately high strength towith stand the pressure differentials, and the melted plastic does notmigrate laterally to an extent which blocks or even substantiallydeforms the small passageways and apertures.

Therefore, regenerators having the configuration of FIG. 11 remain leakfree while providing a high ratio of helium storage volume torefrigerant flow volume. The helium is pressurized as previouslydescribed, so that the mass ratios are many times greater than thevolume ratios. Moreover, because of the small passageways 83 and thesmall wall thicknesses, each small mass of the refrigerant interchangesthermal energy through short path lengths with a much greater thermalstorage mass in the adjacent apertures 85. There is substantiallyinsignificant thermal gradient within each passageway 83,velocity-gradients are extremely low, and the filling factor is high.For such reasons, the low losses and high efficiencies inherent instructures in accordance with the invention are further improved. Forultra low temperature regenerators of maximum efficiency it is preferredto have passageway, wall and aperture dimensions of no greater than 0.1because of the short path lengths and high degree of interspersion thisprovides.

The transverse mesh elements of these heat exchangers need not be inabutment, but can be separated by composite elements which insureretention of strength. A fragmentary view of a'section of aheatexchanger employing one such configuration is illustrated in FIG. 12. Acompletely reinforced wall construction is achieved through theinterspersion of narrow width screen wall elements 89 between theprincipal transverse screen elements 59 with the screen wall elementsbeing built up along a selected length to space the principal elements59. A plastic matrix 90 fills the interstices of the screen wallelements 89 to again form a composite filament-reinforced barrier orpassageway wall. It will be appreciated that the fragment shown in FIG.12 relates only to a fragment of one wall in a passageway system.This-wall construction can also withstand a substantial pressuredifferential, inasmuch as the composite material has substantialcircumferential strength. Conduction across the thickness of the wall bythe presence of the apertured mesh 89 serves to enhance the heattransfer characteristics, although the wall elements can be insulated ifdesired.

The arrangement of FIG. 12 illustrates a different way of shaping thelongitudinal temperature gradient to a particular profile. The greaterspacing of the principal transverse elements 59 (in comparison to theexample of FIGS. 1-4) controls the transfer between two counterflowingfluids, permitting achievement of both extremely shallow and steeptemperature gradients within a single heat exchanger construction.

As shown in the side sectional view of FIG. 13 and the enlargedfragmentary view of FIG. 14, in somewhat idealized form, thecross-sectional areas of separate passageways may also be varied alongthe length of a counterflow exchanger 10' having uniformly interspersedpassageways for two fluids. In the formation and assembly of thelaminate, successive nonconductive elements have successive wallsegments that are slightly displaced relative to each other in apreselected pattern. For a single passageway, each successivenon-conductive element defining a passageway wall overlaps theimmediately adjacent non-conductive elements along a selected distancerelative to the wall thickness. Thus a series of plastic elements mayhave the segments defining one passageway vary progressively in the sizeof their perimeters, but with each overlapping the adjacent elements.When the plastic is heated, melts and subsequently migrates into themesh, the overlapping relationship is preserved and the passageway wallis unified. The wall is continuous and although there is a minuteincremental displacement of the plastic at one screen relative to theadjacent screen, the overall result is the introduction of anessentially continuous curvature into the longitudinal axis.

In FIG. 13 the curvature provides alternate pairs of curved walls 92, 93having oppositely bowed characteristics, providing cross-sectional areasof greatest size in the end regions, and in the center region,respectively. Such an arrangement is useful in compensating flowsbetween respective passageways in a counterflow exchanger, in order toachieve better heat transfer effectiveness by equalizing the separateflows, as described in a co-pending application for patent, owned by theassignee of the present application, entitled Flow Compensator ForExchanger Apparatus, Ser. No. 795,922, filed Feb. 3, 1969, now US. Pat.No. 3,608,629, issued Sept; 28, 1971.

Although there have been described above a number of alternative formsand modifications of structures and methods in accordance with theinvention, it will be appreciated that the invention is not limitedthereto, but encompasses all structures and methods in accordance withthe appended claims.

What is claimed is:

1. A heat exchanger matrix for counterflowing fluids comprising:

v 12 stantially parallel to a selcted flow path axis, said a pluralityof substantially contiguous thermally conductive screen elementsdisposed substantially transversely across desired flow paths; and

heat-flowed plastic barrier means defining walls passageways defining atleast two interspersed sets vidual passageways of each set desirablymaintainof passageways for counterfiowing fluids, the indidefiningpassageways varying in cross-sectional area along their lengths toequalize flows in said flow paths, the variations in area providingpasalong said flow paths, said barrier means impreging substantiallyequal flows, said body including a nating and sealing said screenelements, said barplurality of fine mesh screens substantially transriermeans defining, with the filaments of said verse to the flow path axis,and interior heat-flowed screen elements, composite walls defining thedeplastic means defining said passageways and filling sired flow paths,said screen elements thermally inthe interstices of said screens toprovide filamenterconnecting said flow paths, said composite wallstreinforced composite passageway walls, said passageways of one setbeing concave along their lengths and said passageways of the other setbeing convex along their lengths.

3. The invention as set forth in claim 2 above,

15 wherein said passageway walls have essentially continuous curvaturesdefined by minute incremental displacements of the plastic means'betweensuccessive screen pairs.

sageways for the different counterflowing fluids that are oppositelybowed along their lengths. 2. A high efficiency heat exchanger forcounterflowing fluids comprising:

a longitudinally compressed composite body having selected plurality ofinterior passageways lying sub-

1. A heat exchanger matrix for counterflowing fluids comprising: aplurality of substantially contiguous thermally conductive screenelements disposed substantially transversely across desired flow paths;and heat-flowed plastic barrier means defining walls along said flowpaths, said barrier means impregnating and sealing said screen elemEnts,said barrier means defining, with the filaments of said screen elements,composite walls defining the desired flow paths, said screen elementsthermally interconnecting said flow paths, said composite walls definingpassageways varying in cross-sectional area along their lengths toequalize flows in said flow paths, the variations in area providingpassageways for the different counterflowing fluids that are oppositelybowed along their lengths.
 2. A high efficiency heat exchanger forcounterflowing fluids comprising: a longitudinally compressed compositebody having selected plurality of interior passageways lyingsubstantially parallel to a selcted flow path axis, said passagewaysdefining at least two interspersed sets of passageways forcounterflowing fluids, the individual passageways of each set desirablymaintaining substantially equal flows, said body including a pluralityof fine mesh screens substantially transverse to the flow path axis, andinterior heat-flowed plastic means defining said passageways and fillingthe interstices of said screens to provide filamentreinforced compositepassageway walls, said passageways of one set being concave along theirlengths and said passageways of the other set being convex along theirlengths.
 3. The invention as set forth in claim 2 above, wherein saidpassageway walls have essentially continuous curvatures defined byminute incremental displacements of the plastic means between successivescreen pairs.